Known impregnated coring crowns consist of a head (which performs the actual drilling) in the form of a matrix, prepared by powder metallurgical methods, in which the diamond particles are distributed as homogeneously as possible. In some instances the matrix consists of a bronze. In other cases, the matrix consists of one of, or a mixture of, tungsten, tungsten carbide, nickel and many other metals and refractory carbides infiltrated with a nickel bronze. The matrix depending on its composition may be prepared by either hot-pressing or by infiltration. Under the action of the applied forces on the bit resulting from the drilling machine acting on the rotating drill string behind the bit, the pressure that the individual diamond particles exert on small areas of the rock causes the rock to fragment into small particles. As the drilling proceeds the exposed surface of each diamond particle wears, and the matrix in between is also worn away. Should the matrix wear be insufficient, the active particles of diamond will eventually be worn flush with the matrix and new particles will not be exposed at a sufficently fast rate, drilling will no longer be possible and the bit can be said to have become "closed up".
It is an object of the invention to provide a matrix which will substantially ameliorate the above problems.